Many office chairs come with armrests. It is healthier for the shoulders, which are very mobile joints, not to be continually fixed in position by armrests, however adjustable. Far better to learn to shoulder roll, which encourages a range of motion and optimal arrangement in the joint. Shoulder rolls also help adjacent problem areas such as the trapezius muscle, neck, and upper back, and improve circulation to the area. A well-connected shoulder can support the weight of the arm effortlessly as you do your tasks.
Armrests also create the significant problem of not being able to come close to the keyboard, thus encouraging rounding of the shoulders. The absence of armrests allows an almost cockpit-like feeling of being surrounded by the desk and keyboard with no temptation to migrate the shoulders forward.